Unlike other industrial revolutions from the past, any current technology to be innovative and competitive at the same time should take into account a number of aspects that are crucial due to the current social and political circumstances. These aspects are mainly: [i] A more rational use of raw materials; [ii] Prioritization of the recycling, reuse and recovery of industrial wastes and [iii] Minimizing the production of emissions.
In this sense, it is known that processing of minerals have severe effects on the environment not only for the large consumption of energy, water and chemicals but also because of the big quantities of waste products generated after these industrial activities. Such discarded materials are a source of toxic elements that may be problematic when fulfilling disposal specifications according to local legislations. However, the concern is not exclusively environmental, but economic as well. These waste materials can also be regarded as valuable new-generated raw materials because they are intrinsically generated in large quantities as by-products of existing industrial activities, and they are a rich source of valuable metals that have been concentrated as a result of the mineral process.
As a matter of fact nowadays, producing 1 ton of zinc generates 300 tons of leaching residues (Peng R. Q., Metallurgy of Lead-Zinc, Science Press, Beijing, 2003: 114; and Jiang J. M., Chinese J. Nonferrous Metals, 2004, 14(S1): 51). These leaching residues from electrolytic zinc plants contain high amounts of minor components such as lead, silver, gold and indium and their recovery could represent a substantial improvement in the profitability of a mining or metallurgical project as well as decreasing their environmental impact. This is due to the fact that after treatment of ores and concentrates through conventional extraction processes, the minor components such as lead and silver remain in the process residue. These residues typically assay 15-40% lead as lead sulphate and associated silver which for the most part are considered as unsuitable as feed for a conventional lead smelter except in small amounts.
In the past these leaching residues have been shipped to independent primary smelters for recovery of the metal values contained therein. The cost of shipping and processing the raw residue substantially detracts from the intrinsic value of the residue. For this reason, numerous processes are developed for the recovery of lead and silver from metallurgical residues. They are specially focused on metallurgical techniques, which can be either pyrometallurgical or hydrometallurgical routes (Zhang, Y. et al.; Rare Metals, 2012, 31: 4, 402-406).
In the case of pyrometallurgical processing, when such raw residues (mostly of a sulphated nature) are directly fed, there is high consumption of energy and emissions production. In addition only up to 10% of total silver is recovered, making this technique most of the time unsuitable, (Nakamura T. et al.; Fundamentals of the pyrometallurgical treatment of zinc leach residue, Proc. 2nd International Symposium of Quality in Non-Ferrous Pyrometallurgical. CIM, Montreal, 1995:341).
Regarding hydrometallurgical processing, Farahmand et al. (Hydrometallurgy, 2009, 95: 326) adopted brine leaching of lead-bearing zinc plant residues, wherein the extraction of lead is 89% but silver cannot be recovered. EP 0 042 702 B1 describes a method of leaching lead sulphate containing material in sodium chloride solution and after residue separation, precipitating the Pb from solution with milk of lime. However, the lead precipitates contain considerable amounts of chloride and sulphate and therefore are not acceptable to conventional lead smelters except in small amount and at depressed prices due to the deleterious effects of chlorides.
Within hydrometallurgical routes there have also been described processes wherein the lead is recovered by electrowinning from secondary materials (Frias C. et al.; Lead-Zinc 2000 Symposium, 791-801). Although electrowinning is generally acceptable for the extraction of valuable metals, the high capital cost of the electrolytic systems appear disadvantageous to established smelters when compared with the market price of lead.
Therefore, whilst these methods solve the problem of separating the lead but not always silver from hydrometallurgical residues there is also an additional difficulty in the recovery of the lead and silver from the solution in a usable form. Additionally, these processes are introduced to recover metals one by one, and thus requiring capital intensive equipment and therefore plant.
Consequently, there is a need to provide a process for the simultaneous extraction and recovery of lead and silver into the same product which will be acceptable to conventional lead smelters in high purity, large tonnages and at a premium price.